Hunting the First Hominid

Self-centeredly, human beings have always taken an exceptional
interest in their origins. Each discovery of a new species of
hominid—both our human ancestors and the near-relatives
arising after the split from the gorilla-chimp lineage—is
reported with great fanfare, even though the First Hominid remains
elusive. We hope that, when our earliest ancestor is finally
captured, it will reveal the fundamental adaptations that make us
us.

There is no shortage of ideas about the essential nature of the
human species and the basic adaptations of our kind. Some say
hominids are fundamentally thinkers; others favor tool-makers or
talkers; still others argue that hunting, scavenging or bipedal
walking made hominids special. Knowing what the First Hominid looked
like would add some meat to a soup flavored with speculation and
prejudice.

Genetic and molecular studies provide one sort of insight, showing
what sort of stuff we are made of, and that it is only
slightly different stuff from that which makes up the apes
(gorillas, chimps, orang-utans and gibbons). From the molecular
differences among the genes of humans and apes, geneticists estimate
the time when each of the various ape and hominid lineages diverged
from the common stem. The result is a sort of hairy Y diagram, with
multiple branches instead of simply two as is usual on a Y. Each
terminus represents a living species; each branching point or node
represents the appearance of some new evolutionary trait, such as
new molecules, new genes, new shapes or new proportions of limb,
skull and tooth. Unfortunately, this way of diagramming the results
tends to lull us into thinking (falsely) that all the evolutionary
changes occurred at those nodes, and none along the branches.

Such studies omit a crucial part of our history: the extinct
species. Only the fossil record contains evidence, in context, of
the precise pathway that evolution took. In this technological age,
when sophisticated instrumentation and gee-whiz algorithms seem
downright necessary, the most basic information about the evolution
of our lineage still comes from branches of science that operate in
rather old-fashioned ways. The primary discoveries in paleontology
(the study of old things), paleoanthropology (the study of old
humans) and archaeology (the study of the old stuff that old humans
leave around) still rely on the efforts of a handful of
investigators who slog around on foot or excavate in desolate
landscapes. Fancy equipment can't replace eyes and brains, although
instrumentation plays a crucial role in the dating and analysis of
fossil remains.

Finding the evolutionary origin of hominids is a little like
stalking big game. Paleoanthropologists struggle to establish when
and where their quarry was last seen and what it was
like—hoping to follow its tracks backward in time. (Why
hominids or any other group arose is such a metaphysical
question that most paleoanthropologists run away screaming when it
is asked.)

When the first hominid slinked through the underbush has
been estimated from molecular clocks and confirmed by radiometric
dates. These lines of evidence converge on a period between 5
million and 7 million years ago as the time when a primitive,
perhaps vaguely apelike species evolved some definitive new
adaptation that transformed it into the First Hominid. But like the
point of inflection on a line graph, the first species in any new
lineage is only readily apparent after the fact. The emergence of
the first hominid was probably not obvious in prospect but only now,
in retrospect—in the context of the entire evolutionary record
of the hominids—when the long-term evolutionary trends can be
seen.

Where this dangerous creature once lived has to be Africa,
since both our closest living relatives (chimpanzees and gorillas)
and all early hominids (older than about 2 million years ago) are
African.

What to Look For?

What happened, exactly, andto whom,
remain to be discovered. Two newly discovered fossil species
have each been proposed to be the First Hominid. This circumstance
raises a significant issue: How would we know the First Hominid if
we saw it?

Making a list of key features that differentiate apes from people is
not difficult, but misleading. It is absurd to expect that all of
these differences arose simultaneously during a single evolutionary
event represented by the final fork on the hairy Y diagram. The
first ape on the gorilla-chimp lineage was neither a gorilla nor a
chimpanzee, for modern gorillas and chimps have had at least 5
million years to evolve in isolation before arriving at their modern
form. In the same way, the First Hominid on our lineage was not a
human and did not possess all of the characteristics of modern
humans.

Using a hairy Y diagram, we can limit the number of contenders for
the essential or basal hominid adaptation:

—Wishful thinking aside, hominids are not simply brainy apes.
Alas, until about 2 million years ago no hominid had a brain larger
than an ape's relative to its body size.

—Hominids might be apelike creatures that have lost their
sexual dimorphism. Sexual dimorphism is exhibited as male-female
differences that are not related to reproduction. For example, male
orangs are typically much larger than females and have longer canine
teeth that hone sharper with wear. The fossil record shows that
hominids lost their dental sexual dimorphism first, since all known
hominids have small and flat-wearing canines. In contrast, sexual
dimorphism in body size persisted in hominids until about 2 million
years ago, when the genus Homo first appeared.

—Thick dental enamel may be a key hominid trait. All hominids
have thick enamel, whereas all fossil and living apes (except those
in the orang-utan lineage) have thin enamel. Because the fossil
record of apes is so poor, we do not know whether the primitive
condition for apes and hominids was thick or thin enamel. Indeed,
how enamel thickness is to be measured and evaluated has generated
many pages of debate.

—Hominids are hand-graspers or manipulators (from the Latin
for hand, manus), whereas apes are foot-graspers. These
differences are reflected in the sharp contrasts in the hand and
foot anatomy of apes and humans. Apes have divergent big toes and
long, curved toe bones for holding onto branches; their thumbs are
short and cannot be opposed to the other fingers for skillful
manipulation. Human beings are the opposite, with long, opposable
thumbs and big toes that are closely aligned with the remaining
short, straight toes. Human feet are nearly useless for grasping but
are well adapted to bipedalism. An intermediate condition occurs in
early hominids such as Lucy (the best-known individual of
Australopithecus afarensis), who had opposable thumbs
and numerous adaptations to bipedalism, and yet retained rather long
and curved toes. Lucy and probably other types of
Australopithecus were walkers, hand-graspers and
somewhat compromised foot-graspers.

A description of our desired prey, then, might read like this:

An ape-brained and small-canined creature, with dental
enamel of unknown thickness. Large if male but smaller if female.
May be spotted climbing adeptly in trees or walking bipedally on the
ground. Last seen in Africa between 5 million and 7 million years
ago.

From this description, can we identify the First Hominid? Well,
no—not yet.

A Tale of Two Trophies

Yohannes Haile-Selassie of the University of California, Berkeley,
described a likely contender from Ethiopia in July of 2001. His
material comes from Ethiopian sediments between 5.2 million and 5.8
million years old and is called Ardipithecus ramidus
kadabba, a new subspecies. Ardipithecus means
"root ape" in the Afar language, and the species has been
explicitly proposed to be a "root species" ancestral to
all later hominids.

Haile-Selassie's specimens include more than 20 teeth, some
associated with a mandible or lower jaw; substantial pieces of two
left humeri, or upper arm bones; a partial ulna from the same
forearm as one humerus; a partial clavicle or collarbone; a half of
one finger bone; and a complete toe bone.

No ironclad evidence of bipedality in any Ardipithecus
specimen has yet been published. In this collection, the only
evidence about habitual patterns of locomotion comes from the single
toe bone. Its weight-bearing surface faces downward as in bipeds,
not inward as in apes. Any jury might be suspicious that
Ardipithecus was bipedal, but none of the really telltale
body parts—pelvis, complete femur, tibia, or ankle
bones—has yet been recovered. The preserved bones of the arm,
finger and shoulder closely resemble those of Lucy and may have been
used in grasping and tree-climbing. Ardipithecus is as yet
too poorly known for its relative brain size or sexual dimorphism in
body size to be assessed.

The other candidate that has already been bagged is a 6
million-year-old find from the Tugen Hills of Kenya called
Orrorin tugenensis. Its generic name is derived from
the Tugen language and means "original man"—a claim
as bold as "root ape." Found by a joint French-Kenyan team
headed by Brigitte Senut of the Centre nationale de la recherche
scientifique, the Orrorin fossils include a few teeth, some
embedded in a jaw fragment; a partial humerus; a finger bone; and
substantial parts of three femurs, or thigh bones.

The femurs, which might provide definitive evidence of bipedality,
are incomplete. The sole evidence for bipedality lies in the head of
the femur in Orrorin, which is proportionately larger than
Lucy's. One reason to evolve a large-headed femur is to dissipate
the forces produced by bipedalism. The team concludes that
Orrorin evolved bipedalism separately from Lucy (and from
other species of Australopithecus), making Orrorin
the only known ancestor of Homo. Australopithecus
is displaced to an extinct side-branch of the hominid lineage.

Their surprising conclusion is not universally accepted. Skeptics
reply that the femoral differences between Orrorin and
Australopithecus might disappear if Orrorin's
femur were compared with that of a large male individual rather than
with the diminutive Lucy.

As in Ardipithecus, the bones from the upper limb of
Orrorin show tree-climbing adaptations. Neither relative
brain size nor body size dimorphism can be evaluated in
Orrorin.

Where Orrorin and Ardipithecus differ are in their
teeth. Orrorin appears to have thick enamel, like a hominid
or an orang-utan, and Ardipithecus seems to have thin
enamel, like other apes. This dental comparison might resolve the
question "Who is the First Hominid?" in favor of
Orrorin, except that Orrorin's canine teeth imply
the opposite conclusion. Orrorin's single known canine is
sizable and pointed and wears like an ape's canine. In contrast, the
several known canines of Ardipithecus are all small-crowned
and flat-wearing, like a hominid's canines.

Puzzlingly, Ardipithecus and Orrorin show
different mosaics of hominid and ape features. Both may be bipeds,
although Ardipithecus is a biped in the manner of
Australopithecus and Orrorin is not. If one of
these two newly announced species is the First Hominid, then the
other must be banished to the ape lineage. The situation is
deliciously complex and confusing.

It is also humbling. We thought we knew an ape from a person; we
thought we could even identify a man in an ape suit or an ape in a
tuxedo for what they were. Humans have long prided themselves on
being very different from apes—but pride goeth before a fall.
In this case, embarrassingly, we can't tell the ape from the hominid
even though we have teeth, jaws, and arm and leg bones.

Paleoanthropologists must seriously reconsider the defining
attributes of apes and hominids while we wait for new fossils. In
the meantime, we should ponder our complicity, too, for we have been
guilty of expecting evolution to be much simpler than it ever is.